High-pressure metal halide arc discharge lamps generally comprise an elongated arc tube containing an ionizable fill and having press seals at each end of the tube. Disposed within the arc tube are two main electrodes, one at each end. The electrodes are generally supported in the press seals and are usually connected to a thin molybdenum ribbon, disposed within the press seal, the purpose of the ribbon being to provide an electrical feedthrough of low thermal expansion, owing to its thinness, while having sufficient current carrying capacity, owing to its width.
In order to facilitate starting of the gaseous discharge, a starter electrode may be disposed in the arc tube, adjacent to one of the main electrodes. Such an electrode is used because a discharge can be ignited between the starter electrode and its adjacent electrode at a much lower starting voltage than is required to ignite a discharge between the two main electrodes. Once the discharge is ignited, the ionized gas provides primary electrons between the two main electrodes and if enough potential is available between the main electrodes a discharge will be formed therebetween. The starter electrode normally has a resistor in series with it to limit the current flowing through the starter electrode after the discharge has started.
However, the press sealed electrical feedthrough for the starting electrode suffers a sodium electrolysis failure mechanism which leads to premature seal failure and this is made worse at the elevated seal temperatures associated with the newer low color temperature, high efficiency metal halide lamps. For these reasons, the starter electrode approach has been abandoned in favor of a high voltage starting pulse applied directly to the main electrodes of the arc tube. With this method the seal failure problems associated with the starting electrode have been overcome, however, there is a substantial statistical starting time between the time the high voltage is applied to the lamp electrodes and the gas breakdown time when the discharge occurs. By "statistical" starting time, it is meant that the breakdown or starting time for a give lamp and starting circuit is distributed over a range of values, such that, if the voltage is applied N times, the time at which breakdown occurs is distributed over a relatively wide range indicating that in some specific cases, the starting time is relatively short and in some cases, relatively long.